A composite signal is composed of a carrier wave and a modulation signal of a base signal superimposed on the carrier wave. For example, a micro-electro-mechanical system (MEMS) element of a tuning fork type provided in a gyro sensor vibrates in response to an element driving signal and outputs a composite signal, in which an angular velocity vibration signal is superimposed on the element driving signal. The angular velocity vibration signal vibrates in accordance with an angular velocity of the MEMS element. In this case, the element driving signal is a carrier wave and the angular velocity vibration signal is the modulation signal. The base signal is an angular velocity of the MEMS element.
This composite signal tends to change with factors (low-speed change factors), which changes at relatively low speed, such as temperature and low-frequency noise. For this reason, a demodulation device is required to detect the base signal from the composite signal by removing influence of the low-speed change factors.
JP 2000-283790 A (U.S. Pat. No. 6,255,976) discloses a process of removing from a general sensor signal unnecessary components, which are not a composite signal of a carrier wave and a modulation signal. In this process, unnecessary components included in a sensor output signal, which is a detection object signal, are removed by calculation of a ratio between an analog-digital (AD) conversion value of a predetermined reference signal and an AD conversion value of the sensor output signal.
Further, this process needs an analog switch to select by switching either one of the reference signal or the sensor output signal to be inputted to an AD conversion circuit to calculate the AD conversion value of the reference signal and the AD conversion value of the sensor output signal. This switch changeover operation need more time to finish the process of noise removal.
Further, the AD conversion value of the reference signal and the AD conversion value of the sensor output signal are acquired at different time points. Because of difference in time points of acquisition of the AD conversion values, influence of the low speed change factor is not removed sufficiently.
Further, since not only the analog switch but also an analog switch control circuit need be provided, a circuit size need be increased. Increases in the circuit size and the switch changeover process result in an increase in power consumption.
JP 2015-21782 A discloses a self-exciting resonance circuit for vibrating an element of a gyro sensor.